(1) Field of the Invention
The present invention relates to a packaging material comprising an iron foil. More particularly, the present invention relates to a packaging material excellent in processability, corrosion resistance, storability, appearance characteristics and safety. Furthermore, the present invention relates to a packaging container and a packaging container lid, which are composed of this packaging material.
(2) Description of the Prior Art
As packaging materials heretofore used for containers and lids, there can be mentioned metals, plastics, paper, glass and ceramics and composite materials thereof. In view of the combination of the barrier property to gases such as oxygen, carbon dioxide gas and water vapor and the mechanical strength, metal materials are most preferred.
However, disposal of metal cans or metal lids by incineration is difficult and there arises a problem of so-called can pollution. Accordingly, laminates composed of a metal foil and a resin film are used as packaging materials that can be easily disposed of in the field of sealing containers or sealing lids. Almost all of commercially available laminates for containers or lids comprise an aluminum foil as the substrate. Although the aluminum foil is excellent in the appearance characteristics and flexibility, corrosion such as pitting or peeling of the coating layer is readily caused by a content containing a salt such as sodium chloride at a relatively high concentration or a content containing an organic acid even though the surface is coated with an organic resin, and defects such as leakage of the content and reduction of the storability are brought about.
As the metal foil, there can also be mentioned foils composed mainly of iron or steel, such as pure iron foils, steel foils and tinplate foils. However, many problems should be solved when these foils are used as packaging materials for foods. Iron or steel is a metal which is likely to rust, and rusting is readily caused in the process for the production of a packaging material or during the storage and the appearance or commercial value is drastically degraded by rusting. Furthermore, dissolution of iron or incorporation of rust results in drastic degradation of the content flavor-retaining property.
Furthermore, since the thickness of an iron or steel foil is much smaller than that of a steel plate, the iron foil or steel foil is very poor in the draw processability or formability, and wrinkles are formed at the drawing step or the foil is broken during the processing, with the result that formation of a container becomes difficult. This defect becomes prominent when a relatively thick organic resin coating is formed so as to improve the corrosion resistance or the rusting resistance.
More specifically, it was found that if the thickness of a steel sheet is reduced below 0.12 mm and a laminate of this steel sheet and an organic resin coating layer is formed into a drawn container, the following problems arise. In the first place, wrinkles are readily formed in the part ranging from the flange portion to the side wall at the draw-forming step, and because of the presence of these wrinkles, attainment of sure sealing by heat sealing or wrap seaming becomes difficult. In case of a container in which formation of wrinkles is prevented, however, if the container is stored for a long time after filling of the content and sealing, corrosion below the organic resin coating is advanced in the inner surface portion of the container, especially in the side wall portion, is advanced, and the life of the product is drastically shortened by reduction of the gas barrier property or dissolution of the metal. The reason is that if the wrinkle controlling pressing force is increased so as to reduce formation of wrinkles at the step of forming the container, the organic resin coating on the inner surface or the bonding interface between the resin coating on the inner surface and the steel foil is damaged or even if not damaged, a stress causing the peeling is left.
Another important problem are sides in that the iron foil has a larger rigidity than the aluminum foil and the thickness of the iron foil generally corresponds to the thickness of a sharp blade edge, and therefore, when the finger tip touches the cut edge of the iron foil, the finger tip is readily cut. Moreover, even if the surface of the iron foil is carefully coated and protected, rusting is readily caused from the exposed cut edge.